Research Article www.acsami.org
Superior Mechanical Properties of Epoxy Composites Reinforced by 3D Interconnected Graphene Skeleton Ya Ni, Lei Chen, Kunyue Teng, Jie Shi, Xiaoming Qian, Zhiwei Xu,* Xu Tian, Chuansheng Hu, and Meijun Ma Key Laboratory of Advanced Braided Composites, Ministry of Education, School of Textiles, Tianjin Polytechnic University, Tianjin 300387, P. R. China S Supporting Information *
ABSTRACT: Epoxy-based composites reinforced by three-dimensional graphene skeleton (3DGS) were fabricated in resin transfer molding method with respect to the difficulty in good dispersion and arrangement of graphene sheets in composites by directly mixing graphene and epoxy. 3DGS was synthesized in the process of self-assembly and reduction with poly(amidoamine) dendrimers. In the formation of 3DGS, graphene sheets were in good dispersion and ordered state, which resulted in exceptional mechanical properties and thermal stability for epoxy composites. For 3DGS/epoxy composites, the tensile and compressive strengths significantly increased by 120.9% and 148.3%, respectively, as well as the glass transition temperature, which increased by a notable 19 °C, unlike the thermal exfoliation graphene/epoxy composites via direct-mixing route, which increased by only 0.20 wt % content of fillers. Relative to the graphene/epoxy composites in direct-mixing method mentioned in literature, the increase in tensile and compressive strengths of 3DGS/epoxy composites was at least twofold and sevenfold, respectively. It can be expected that 3DGS, which comes from preforming graphene sheets orderly and dispersedly, would replace graphene nanosheets in polymer nanocomposite reinforcement and endow composites with unique structure and some unexpected performance. KEYWORDS: three-dimensional graphene skeleton, epoxy-based composites, ordered arrangement, resin transfer molding, mechanical properties
1. INTRODUCTION Graphene, with one-atom thick and two-dimensional (2D) sheet of sp2-bonded carbon, has a range of excellent properties.1,2 Since Geim et al. peeled a few graphene sheets from highly crystalline graphite by a “scotch tape” method in 2004,3 some unique properties of this conceptual matter have been found and indicate potential applications in nanocomposites with various matrixes, ultrathin membrane materials, etc.4 Owing to its exceptional physical properties, such as high surface area, excellent thermal conductivity, extremely high mechanical flexibility, and electrical conductivity, graphene has attracted tremendous research interest in recent years.5−8 The reliable and scaled-up production of graphene derivatives, such as graphene oxide and reduced graphene oxide, offers a wide range of possibilities to synthesize graphene-based functional materials for various applications. These excellent characteristics have brought great achievements in design of novel graphene functional materials with tailored properties.9 Individual graphene sheet has prominent mechanical properties, and a single-layered graphene is found to exhibit extraordinary Young’s modulus of 1100 GPa and tensile strength of 130 GPa.10,11 These characteristics make them ideal fillers for polymer nanocomposites that were used in advanced materials.1,12 Thus, incorporating graphene sheets into © 2015 American Chemical Society
composite materials is one feasible route to harness these properties for applications.8,11 In the past few years, graphene sheets have been introduced into a wide range of polymer matrix, including epoxy, polystyrene, polypropylene, and so on, for various functional materials.13 Epoxy, one of the most important thermosetting polymers, is used in widespread fields. Numerous attempts such as mixing epoxy polymers with a second phase of superior nanofillers (nanospheres, nanotubes, nanoplatelets, etc.) and their combination have been made to improve the toughness, stiffness, and strength and to endow the multifunctional properties.14 Graphene/polymer composites are mostly prepared with the methods of solvent mixing and in situ polymerization, which often consume large amounts of organic solvents. However, the bottleneck is that graphene or graphene oxide sheets tend to form severe aggregations in composites due to the strong van der Waals forces among them, which limits load transfer from the matrix to the sheets.15 The strong van der Waals force between the fillers results in uneven dispersion, and consequently, the properties of composites fall short of the expectations. Thus, to achieve Received: March 23, 2015 Accepted: May 7, 2015 Published: May 7, 2015 11583
DOI: 10.1021/acsami.5b02552 ACS Appl. Mater. Interfaces 2015, 7, 11583−11591
Research Article
ACS Applied Materials & Interfaces
Figure 1. Flowchart for the fabrication process of EG/epoxy composites (a) and 3DGS/epoxy composites (b).
supercapacitors,34 adsorption,35 catalysis supports,36 and sensors.37 Similarly, new ideas have been brought to the preparation of polymer composites. Several researchers, JangKyo Kim et al.38,39 and Kin Liao et al.,40 have mixed 3D graphene network with polymer using a vacuum-infiltration method. Nevertheless, the performance of its composites was much less than expected on account of the insufficient in formation methods of polymer composites and the damage of cured composites in the process of etching template. Herein, taking into account its special structure and exceptional properties, we incorporated 3D graphene network into the fabrication of epoxy-based composites as preforming reinforcement. First, 3D graphene skeleton (3DGS) was prepared by self-assembly and in situ reduction method, in which poly(amidoamine) (PAMAM) dendrimer acted as reductant and cross-linking agent. Then resin transfer molding (RTM) method was applied to fill the porous structure of 3DGS with epoxy, in which epoxy had better effect on flow patterns and infiltration under the action of external pressure compared with vacuum-infiltration method, which depended on the mobility of epoxy itself. In this way, we developed a novel method of fabricating graphene/epoxy composites, which was significantly different from traditional approach based on directly mixing graphene sheets and epoxy. It is expected that the aggregation of graphene sheets can be effectively restrained and that graphene sheets even act as ordered support structure in 3DGS/epoxy composites, which is completely different from the messy arrangement for graphene sheets in graphene/epoxy composites with direct-mixing method.
optimal enhancement in the properties of graphene/polymer composites, several key issues should be resolved, namely, improved dispersion and exfoliation of graphene,16 alignment of graphene in polymer,17,18 surface modification of graphene, and interfacial adhesion with the matrix.19,20 On one hand, different dispersion methods as well as filler surface functionalization and the use of a variety of dispersing agents were developed to overcome these challenges.21 Tremendous approaches have also been exploited to functionalize graphene or graphene oxide sheets to fabricate derivatives, which may improve their compatibility in a polymeric matrix. Unfortunately, these methods are still unable to significantly improve the performance of polymer composites. Homogeneous and stable dispersion of the nanofillers in the polymer matrix to enhance the comprehensive performance of composites remain critical challenges in this field.22 On the other hand, alignment and rational assembly of graphene sheets have attracted a great deal of attention for the preparation of composite materials.23 In this regard, some methods have been studied, such as electrical/magnetic fields or mechanical shear.24 However, the process can be difficult to achieve, and graphene sheets are in the in-plane direction, which retains aligned morphology as well as limits final applications.22,23 Moreover, the improvement of the overall performance of composite materials is considerably restricted. Hence, to realize reasonable and orderly arrangement of graphene in polymer composites is still a huge challenge for the fabrication of high-performance and multifunctional composites. To improve the dispersion and orientation of graphene sheets, efforts have been made to assemble individual graphene sheets into three-dimensional (3D) graphene network materials such as hydrogels,25 aerogels,5,26 and macroporous films.27 Current methods to prepare 3D graphene macrostructures involve template-directing method,28,29 cross-linking method,30 directed chemical vapor deposition,31 and in situ reduction assembly method.32 These 3D graphene network materials have shown a thriving development due to their excellent performance, such as open porosity, high pore connectivity, large surface areas, certain strength, etc.33 Therefore, the 3D graphene network with unique structure provides a new platform for many applications such as energy storage,
2. EXPERIMENTAL SECTION 2.1. Materials. Natural graphite flakes were supplied by Qingdao AoKe ShiMo Co. Ltd., with an average diameter of 10 μm. PAMAM was supplied by Weihai Chenyuan Co.Ltd.. Epoxy (JC-02A) with an epoxy value of 0.51−0.53 was obtained from ChangShu Jiafa Chemical Co.Ltd. Tetrahydrophthalic anhydride, which was purchased from Wenzhou Qingming Chemical, was used as the curing agent. Acetone, hydrochloric acid (HCl, 30%), potassium permanganate, concentrated sulfuric acid (H2SO4, 98%), and phosphoric acid (H3PO4, 85%) were purchased from Tianjin chemical factory and used as received. 2.2. Preparation of Three-Dimensional Graphene Skeleton. Graphite oxide was synthesized from natural graphite flakes using a 11584
DOI: 10.1021/acsami.5b02552 ACS Appl. Mater. Interfaces 2015, 7, 11583−11591
Research Article
ACS Applied Materials & Interfaces modified Hummers method,41,42 giving graphite oxide in a state of the pale yellow colloid. Graphene oxide was prepared from graphite oxide with ultrasonication. The concentration of graphene oxide in the suspension was determined by freeze-drying a small amount of suspension and then weighing the dried graphene oxide. PAMAM was dissolved in water and added into the graphene oxide suspension, in which graphene oxide (3 mg/mL) and PAMAM were in mass ratio of 1:1. Subsequently, the mixture was stirred for 5 min and then placed for 2 h without stirring to get homogeneous mixture. With the highspeed centrifugal process, a sticky mixture was obtained. With rapid freezing in liquid nitrogen and freeze-drying for 24 h, the porous brown solid was prepared. After heating for 12 h at 150 °C, the resulting 3DGS was fabricated in the amidation and reduction process. And intact structure of 3DGS was still maintained in the ultrasonic process for 3 h. 2.3. Fabrication of Epoxy Composites Reinforced by ThreeDimensional Graphene Skeleton and Thermal Exfoliation Graphene. Preparation of 3DGS/epoxy composites was done following the protocol as shown in Figure 1b: (1) The prepared porous 3DGS was put into the mold, which was sealed securely. (2) Epoxy with 70 wt % curing agent and 1 wt % 2-ethyl-4methylimidazole was mixed and degassed under vacuum at 60 °C for 30 min. (3) The prepared epoxy was injected into the mold by RTM method, and epoxy was in almost constant temperature during the RTM process. (4) The mold was put into oven to cure the composites, followed by a postcuring step at 90 °C for 3 h, 120 °C for 3 h, and 150 °C for 5 h.43 The final 3DGS/epoxy composites were prepared for the following tests. As shown in Figure 1a, thermal exfoliation graphene (EG)/epoxy nanocomposites were prepared by dispersing EG in acetone with ultrasonication for 1 h and adding preheated epoxy. The mixture was homogenized by continued ultrasound for 3 h with the temperature of 60 °C. After acetone removal, the mixture was added with 70 wt % curing agent and 1 wt % 2-ethyl-4-methylimidazole. Then the completed curing process was performed as above. 2.4. Characterizations. Morphology and microstructure of the prepared 3DGS and fracture surface of the composites was observed with a Hitachi S-4800 field-emission scanning electron microscope (FE-SEM) at an accelerating voltage of 10 kV. The reduction process of 3DGS was determined by X-ray Diffraction (XRD, Bruker D8 Discover) with Cu Kα radiation (λ = 1.540 59 Å) with a scanning speed of 5°/min from 3 to 40° and by Fourier transform infrared spectroscopy (FTIR, Bruker Tensor 37) in the 4000−800 cm−1 wavenumber range. The chemical composition was analyzed by Xray photoelectron spectroscopy (XPS, PHI 5700) with Al Kα excitation radiation. Dynamic mechanical thermal analysis (DMA) was performed on a dynamic mechanical thermal analyzer (MettlerToledo 861e instrument) in three-point bending mode at a frequency of 1 Hz with a constant heating rate of 5 °C/min ranging from 50 to 200 °C. Specimen dimensions were 60 mm × 10 mm × 4 mm. Thermal characterization of epoxy and its composite was also performed by a DSC 7 PerkinElmer instrument calibrated with indium under nitrogen atmosphere at the rate of 5 °C/min. Tensile, compressive, and flexural tests were performed to determine the effect of EG and 3DGS reinforced epoxy on the Instron mechanical testing machine, according to the ASTM D638, ASTM C365, and ASTM D790-03, respectively.
Figure 2. (a) Digital image of 3DGS. (b, c) FE-SEM images at different magnifications and (d) broken end of 3DGS.
dehydration process of graphene oxide and PAMAM, the mixture was formed into a microporous monolithic graphene oxide−PAMAM, in which PAMAM played a supporting role. Graphene sheets were to be cross-linked with dendrimer structure caused by thermal reduction in vacuum, in which PAMAM had good reactivity and compatibilization effect due to the multiple end-reactive groups. Under the action of chemical bonds between the graphene sheets and PAMAM, the graphene sheets were inhibited to be restacked and formed a 3D graphene skeleton with open micropores.44 To bear out the stability of this porous structure, 3D graphene skeleton was in ultrasonic processing for 3 h with the solvent of N,Ndimethylformamide, which could disperse graphene sheets and thermally treated PAMAM. The 3DGS was free of damage after ultrasonic treatment (Supporting Information, Figure S2), which indicated that chemical bonds were in existence between graphene sheets and PAMAM. As the structure was supported by assembled graphene sheets, there was noticeable fold on graphene sheets. XRD is a powerful tool to characterize the structural quality of graphite oxide, graphene oxide−PAMAM, and 3DGS materials, as shown in Figure 3, which reveals the information
3. RESULTS AND DISCUSSION 3.1. Morphology and Structure of Three-Dimensional Graphene Skeleton. In a typical synthesis procedure of 3DGS, brown mixture of graphene oxide and PAMAM was transformed to self-assembly porous graphene oxide−PAMAM after freeze-drying. With the reduction process, a black freestanding 3D graphene skeleton was formed, which displayed a macroporous structure less than 100 μm as shown in Figure 2. The distorted graphene sheets were randomly cross-linked with rich micropores interpenetrating inside, and the surface area of 3DGS was up to 202 m2/g.35,37 Under the mixing and
Figure 3. XRD patterns of graphite oxide, graphene oxide−PAMAM, and 3DGS. 11585
DOI: 10.1021/acsami.5b02552 ACS Appl. Mater. Interfaces 2015, 7, 11583−11591
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ACS Applied Materials & Interfaces
bending vibration appeared in graphene oxide−PAMAM sample. For comparison, in the control experiment, 3DGS showed the appearance of new peak in the spectrum around 1400 cm−1 with heat treatment for 12 h, implying the formation of chemical bonds resulting from the reaction between the carboxylic groups and the amine groups,51 while the peaks of oxygen-containing groups reduced significantly associated with the reduction of graphene oxide. 3.2. Structure of Three Dimensional Graphene Skeleton/Epoxy and Thermal Exfoliation Graphene/ Epoxy Composites. The fabrication process for 3DGS/ epoxy composites and thermal EG/epoxy composites are illustrated in Figure 1. FE-SEM images of fracture surface for 3DGS/epoxy composites and EG/epoxy composites are shown in Figure 5. Compared to EG/epoxy composites prepared via
on the structural evolutions. The XRD pattern of graphite oxide exhibited a layer-to-layer stacking distance of 0.81 nm, corresponding to a typical diffraction peak at 2θ of 10.8°, pointing to an ordered structure.42 However, with the process of sonication for graphite oxide and mixing with PAMAM, the characteristic peak of graphite oxide did not appear in the pattern of the graphene oxide−PAMAM sample, clearly indicating that the graphite oxide was fully exfoliated into graphene oxide sheets and that dendrimer PAMAM molecules were well-dispersed in graphene oxide solution and mixed evenly with graphene oxide sheets.45,46 Nevertheless, the disappearance of the peak at 10.8° was also a mark for the reduction of graphene oxide in the graphene oxide−PAMAM sample.47−49 As shown in Supporting Information, graphene oxide separated from graphene oxide−PAMAM sample was partially reduced due to the amine groups in PAMAM, which was confirmed by XPS and XRD (Supporting Information, Figure S1). For the macroscopic condition of graphene oxide− PAMAM, it gave a porous structure after the process of freezedrying. Probably, graphene oxide sheets contacted one another due to π−π stacking and hydrogen bonding on one hand; on the other hand PAMAM monomers adsorbed on the surface of graphene oxide sheets via favorable interactions (e.g., hydrogen bonding) between the amines of PAMAM and oxygencontaining groups of graphene oxide in homogeneous solution of graphene oxide and PAMAM.50 With the heating reaction in vacuum, XRD patterns of 3DGS were similar to the graphene oxide−PAMAM; this result showed a structural stability, where PAMAM played a role to support. The appearance of a broad peak from 15 to 25° may be an indication of the restacking of graphene sheets and PAMAM. The FTIR analysis of graphite oxide and graphene oxide− PAMAM, demonstrated in Figure 4, exhibited the presence of
Figure 5. Fracture surface of 3DGS/epoxy composites by RTM method and EG/epoxy composites via direct-mixing route: (a) EG/ epoxy composites, (b) 3DGS/epoxy composites, (c) polished fracture surface of 3DGS/epoxy composites, (d) polished fracture surface of 3DGS/epoxy composites (after etching process).
direct-mixing route (Figure 5a), the section of 3DGS/epoxy composites by RTM method (Figure 5b) was obviously rough and showed clearly successive profile. Because of the difference in hardness between graphene and epoxy, a certain height gradient appeared between the outline of 3DGS and epoxy under the process of polishing. As shown in Figure 5c, the outline structure of 3DGS in composites where there was in a bright color unfolded evidently after the polishing process. Because of the multilevel arrangement of pore structure in 3DGS, the definition of outline structure under FE-SEM varied (Figure 5c). To reveal the existing state of graphene sheets in 3DGS/epoxy composites clearly, the 3DGS was subjected to a slight etching damage with acid, and the polished surface is shown in Figure 5d. The outline structure in a dark color obviously presented between the blocks of epoxy. The outline structure was interconnected with average sizes in the range of several to dozens of micrometers, resembling the profile of the porous 3DGS.52 As seen in Figure 5, no pores were shown between 3DGS and epoxy or even epoxy resin itself, indicating that 3DGS adhered well to epoxy matrix and that epoxy showed excellent permeability into 3DGS.53 During the preparation of 3DGS/epoxy composites by RTM method, the interconnected 3D graphene skeleton remained intact in epoxy matrix from Figure 5, and damaged phenomenon in
Figure 4. FTIR spectra of graphite oxide, graphene oxide−PAMAM, and 3DGS.
strong peaks corresponding to oxygen-containing groups, such as 1704 cm−1 (CO stretching vibration for carbonyl and carboxylic groups) and 1620 cm−1(CC in aromatic ring). The broader peak around 3405 cm−1 was assigned to hydroxyl groups.36 When water was removed by freeze-drying from the mixture of graphene oxide and PAMAM, graphene oxide− PAMAM with porous structure was obtained, and a new and tiny peak around 1570 cm−1 corresponding to the N−H 11586
DOI: 10.1021/acsami.5b02552 ACS Appl. Mater. Interfaces 2015, 7, 11583−11591
Research Article
ACS Applied Materials & Interfaces
Figure 6. Mechanical properties of composites with different filler loadings: (a) tensile strength, (b) tensile stress−strain curves, (c) compressive strength, (d) flexural strength and modulus.
Table 1. Mechanical Properties of Epoxy Composites Reinforced by Graphene, Graphene Derivatives, and 3DGS
a
filler type
process
TEG graphene GNPs EGNPs graphene RGO GO GO GO DGEBA-f-GO silane-f-GO FGNs a-TEG FGS G-Si GO-D230 GAs GF EG 3DGS
sonication sonication stir and shear sonication and shear sonication sonication and shear sonication and shear shear and shear sonication and shear sonication and shear sonication and shear sonication sonication sonication shear and shear homogenization infiltration infiltration sonication RTM
increase in tensile strengtha (%)
increase in flexural modulusa (%)
30 8
20 ∼10 ∼7 ∼13 ∼12.5
∼0 ∼14 ∼10 ∼28 ∼75 ∼48 21 43 0 ∼24 ∼25
increase in compressive strengtha (%)
∼26
14.8
∼50
